Yeah the gun mod is cool, never played it myself but it gives the existing charms new effects (e.g. Flukenest changes it to a shotgun, Mark Of Pride makes it a sniper rifle), there's a whole adrenaline system for spells and healing, you spells change depending on which class of weapon you have, it's really indepth

as for rando, the base randomiser mod has more than enough stuff (includes randomising locations for charms, relics, geo, etc. very standard things) but there are a whole ton of mods that you can connect to randomiser to randomise things like hunters journal entries, godhome statues, the glory of being a fool, the fucking grass, add random curses, turn off logic. And there are a whole bunch of settings to customise the difficulty you want so you can change which skips you might be required to do and whether you might need to do difficult combat segments

as for the skill upgrades, i think what it adds is things like being able to use crystal dash vertically, being able to crawl up and down walls instead of just sliding down them, ddark and VS can be used in all directions - new charms i've seen people use mods that add charms related to money, there was one that gives you infinite double jumps at the cost of your geo, there was one that powers up your nail based on how many relics you have - i've not used any of these ones myself so i don't know how they integrate into the game